//
// Description : Array and textureless GLSL 2D simplex noise function.
//      Author : Ian McEwan, Ashima Arts.
//  Maintainer : stegu
//     Lastmod : 20110822 (ijm)
//     License : Copyright (C) 2011 Ashima Arts. All rights reserved.
//               Distributed under the MIT License. See LICENSE file.
//               https://github.com/ashima/webgl-noise
//               https://github.com/stegu/webgl-noise
//

float simplex( vec2 v ) {

    const vec4 C = vec4( 0.211324865405187,  // (3.0-sqrt(3.0))/6.0
                        0.366025403784439,  // 0.5*(sqrt(3.0)-1.0)
                        -0.577350269189626,  // -1.0 + 2.0 * C.x
                        0.024390243902439 ); // 1.0 / 41.0
    // First corner
    vec2 i  = floor( v + dot( v, C.yy ) );
    vec2 x0 = v - i + dot( i, C.xx );

    // Other corners
    vec2 i1;
    //i1.x = step( x0.y, x0.x ); // x0.x > x0.y ? 1.0 : 0.0
    //i1.y = 1.0 - i1.x;
    i1 = ( x0.x > x0.y ) ? vec2( 1.0, 0.0 ) : vec2( 0.0, 1.0 );
    // x0 = x0 - 0.0 + 0.0 * C.xx ;
    // x1 = x0 - i1 + 1.0 * C.xx ;
    // x2 = x0 - 1.0 + 2.0 * C.xx ;
    vec4 x12 = x0.xyxy + C.xxzz;
    x12.xy -= i1;

    // Permutations
    i = mod289( i ); // Avoid truncation effects in permutation
    vec3 p = permute( permute( i.y + vec3( 0.0, i1.y, 1.0 ) )
        + i.x + vec3( 0.0, i1.x, 1.0 ) );

    vec3 m = max( 0.5 - vec3( dot( x0, x0 ), dot( x12.xy, x12.xy ), dot( x12.zw, x12.zw ) ), 0.0 );
    m = m*m ;
    m = m*m ;

    // Gradients: 41 points uniformly over a line, mapped onto a diamond.
    // The ring size 17*17 = 289 is close to a multiple of 41 (41*7 = 287)

    vec3 x = 2.0 * fract( p * C.www ) - 1.0;
    vec3 h = abs( x ) - 0.5;
    vec3 ox = floor( x + 0.5 );
    vec3 a0 = x - ox;

    // Normalise gradients implicitly by scaling m
    // Approximation of: m *= inversesqrt( a0*a0 + h*h );
    m *= 1.79284291400159 - 0.85373472095314 * ( a0 * a0 + h * h );

    // Compute final noise value at P
    vec3 g;
    g.x  = a0.x * x0.x + h.x * x0.y;
    g.yz = a0.yz * x12.xz + h.yz * x12.yw;
    return 130.0 * dot( m, g );

}
